The present invention relates to a method for vaporizing and supplying an organometal compound and an apparatus for carrying out the method and more specifically to a method and an apparatus for vaporizing and supplying an organometal compound used as a starting material for manufacturing an epitaxial thin film of a compound semiconductor according to the metal organic chemical vapor deposition (MOCVD) technique.
The MOCVD technique has become of major interest recently as a means for forming, on the surface of a substrate, thin film semiconductors of compound or mixed crystal of III-V group elements or II-IV group elements. According to the MOCVD technique, a crystalline thin film is formed by thermally decomposing an organometal compound such as (CH.sub.3).sub.3 Ga or (CH.sub.3).sub.3 Al as a starting material and depositing the resulting vapor of the decomposed organometal compound on to the surface of a substrate. In particular, this technique makes it possible to form a thin film having uniform thickness on a substrate having a large surface area and is excellent in mass-productivity, controllability of the film composition or the like.
When an organometal compound is employed in the MOCVD technique, a carrier gas such as hydrogen gas is bubbled through the organometal compound so that it comes in contact with the compound to give a carrier gas stream saturated with the vapor of the organometal compound and the carrier gas is guided to a chamber for growing crystals in which a crystal thin film grows on a substrate. In this respect, the MOCVD technique is roughly divided into two groups, one of which is called "normal pressure MOCVD" and the other of which is called "low pressure MOCVD" depending on the pressure established in the crystal-growth chamber. In either of these techniques, the pressure at the both of carrier gas inlet and outlet of a container for the organometal compound as the starting material is adjusted with a valve so that it is equal to 1 atm.
There has conventionally been known an apparatus which is shown by a sectional view in FIG. 6 as such an apparatus for vaporizing an organometal compound and supplying the resulting vapor thereof to a crystal growth chamber, in which a carrier gas of this kind is employed. In FIG. 6, the numerical order 1 represents a container for a carrier gas such as H.sub.2 gas; 2 represents a valve for reducing pressure; 3 represents a massflow controller for controlling the massflow of the carrier gas; 4 represents a liquid organometal compound; and 5 represents a cylinder cabinet which is filled with such a liquid organometal compound 4. The numerical order 6 represents a constant temperature oven; 7 represents an inlet valve; and 8 represents a tube for introduction (a dipping tube) which serves to introduce the carrier gas into the lower portion of the cylinder cabinet 5. The numerical order 9 represents an outlet valve; and 10 represents a needle valve which serves to maintain the pressure in the vicinity of an inlet and an outlet of the cylinder cabinet 5 at about 1 atm. The numerical order 11 represents a crystal growth chamber for growing crystals; 12 represents a heater; 13 represents a substrate; and 14 represents a vacuum pump.
In case of this apparatus to be employed for manufacturing a thin film of a semiconductor, first the temperature of the constant temperature oven 6 is correctly controlled to thus determine the vapor pressure of the organometal compound 4. Then the carrier gas, of which massflow has been precisely controlled by the massflow controller 3, is introduced into the cylinder cabinet 5 by opening the valve 7 and, thereafter, the valve 9 is opened to guide the carrier gas containing the vaporized organometal compound in a desired concentration towards the crystal growth chamber 11. A reduced pressure is established in the crystal-growth chamber 11 by the vacuum pump 14. Thus, an epitaxial thin film of a semiconductor resulting from the organometal compound grows on the surface of the substrate 13.
However, in case of using an organometal compound in a solid state at an ordinary temperature as a starting material for a semiconductor in such as the above explained apparatus. The more the amount of the solid organometal compound charged in the cylinder cabinet 5 decreases, the more the composition of the resulting compound semiconductor varies and deviates from the desired one. Therefore, semiconductor thin films of only unacceptable crystallinity would be obtained. This is because, in case of a solid organometal compound to be employed as a starting semiconductor material, the contact between the carrier gas and the organometal compound takes place on a limited area, i.e., simply on the surface of the solid unlike the case wherein a liquid organometal compound is used and, as a result, the concentration of the organometal compound in the carrier gas does not reach that corresponding to the saturated vapor pressure thereof. Moreover, it is also assumed that since the efficiency of the heat exchange between the organometal compound and the constant temperature oven varies depending on the flow rate of the carrier gas, a temperature difference arises between the oven and the organometal compound and thus a desired amount of vapor cannot be supplied to the crystal growth chamber.
To enhance the efficiency of contact between a solid organometal compound and a carrier, there have been proposed a variety of methods or ideas. For instance, a method in which solid organometal compounds are employed in the form of powder or needles is disclosed in Japanese Patent Provisional Publication 63-55194 and 63-11598. Besides, Collected Resume of 49th Meeting of Society of Applied Physics, 5P-W-7, p. 237 discloses that a porous filter for dispersing a carrier gas is disposed at the apex of dip tube inserted in a cylinder cabinet.
However, in either of these cases, it is impossible to precisely control the quantity of the vaporized organometal compound at the outlet of the cylinder within a variation ranging from 5 to 10%. In particular, in case of using organometal compound in the form of powder as in Japanese Patent Provisional Publication 63-55194 and 63-11598, it is necessary to introduce such powder into the cylinder cabinet through a blank plug or the like after pulverizing the solid organometal compound externally since it is very difficult to make pulverization within the cylinder cabinet. Besides, the organometal compounds are in general highly toxic and highly reactive with oxygen and water. For this reason, a special installation to isolate from external atmosphere is needed to introduce the powder into the cylinder cabinet and, further, the powder as the starting material is often contaminated during pulverization and charging operations.
It is also necessary to maintain the pressure in the vicinity of the carrier gas inlet and outlet for the cylinder cabinet at ordinary pressure even if crystals grow according to the low pressure MOCVD technique. For this reason, it is needed to continuously perform fine adjustment of the needle valve 10 as shown in FIG. 6 which illustrates the structure of a conventional apparatus for vaporizing an organometal compound and supplying the resulting vapor thereof to a crystal growth chamber. The control of the amount of the vaporized organometal compound thus requires a great deal of labor. Furthermore, it is difficult to perform an instantaneous interruption of the supplying of the starting materials and rapid switching of them. Therefore, the sharpness of the crystal growing boundary is greatly impaired.